Practical Approaches to Controlling Bitterness in Homebrews
The goal of homebrewing should be the production of unique creations that are fresh, flavorful and absent of defects. Secondarily, many homebrewers attempt to hit target values of gravity, bitterness and color based on a beer’s recipe or homebrew contest style guidelines. Too often homebrewers obsess over hitting their targets and deem themselves failures if the beer does not measure up exactly to its specifications.
Hop bitterness is one component that homebrewers seek to manipulate, but cannot precisely control. Yet, with an awareness of the relevant variables and the methods to manage them, the homebrewer can successfully produce a beer that contains bitterness within the desired range.
There are formulas to predict the bitterness in beer. However, these equations can lead to a false sense of accuracy because the homebrewer cannot directly measure many of variables that go into these equations nor the final value being calculated (IBUs). Therefore, homebrewers should focus less on the number of decimal places in their formulas and more on consistently repeating the procedures that influence bitterness. Consistency in using fresh hops, creating the same boiling intensity, boiling the hops for the planned length of time and taking the same amount of time chill the wort will enhance a homebrewer’s ability to get a predictable amount of bitterness.
What is an IBU?
Bitterness is usually described by the term “International Bitterness Units” (IBU), which is defined as the milligrams of isomerized alpha acids in one liter of beer. Homebrewers seeking to predict the number of IBUs in their beer usually use some form of the following equation:
where Woz is the weight (in ounces) of the hops; AA is the alpha acid rating of the hops expressed as a decimal (i.e. if the hops contain 7% alpha acids, AA = 0.07); U is the utilization percent, also expressed as a decimal (i.e. 25% utilization means U = 0.25) and Vgallons is the volume of wort, in gallons. (The number 7,490 is a conversion factor that shows up when English units are used.) Thus, to control bitterness in beer, we need to focus on controlling the variables in the equation.
Weight of hops
Most homebrewers measure their hops on a kitchen scale calibrated in units of 1/4 of an ounce. Given that most hop additions are at most a few ounces, there is room for significant differences in IBUs to arise from small, unmeasurable differences in the weight of the hops. If your beer requires a 1 ounce addition of hops, your kitchen scale will likely read “1 oz.” for all weights from just over 7/8 (0.875) ounce to just under 1 1/8 (1.125) ounces. For a 10% alpha acid hop, this range corresponds to a difference of 9 IBUs in your beer! Also keep in mind that just because your bag of hops says “1 oz.” on it, does not mean that the hops weigh one ounce. The guy at the homebrew shop who packaged the pellet hops can make a mistake, too. And finally, it should go without saying that your scale needs to be calibrated or your measurements can be considerably off.
Alpha Acid Content
Hop growers measure the level of alpha acid and express it as a percentage reported to one decimal place (5.8%, for example). The producer-supplied alpha acid rating represents the alpha acid rating at the time of harvest.
However, alpha acid levels are subject to deterioration during storage from the effects of heat, light, air and humidity. Furthermore, some hop varieties are subject to faster deterioration than others are. Hop producers use the “hop storage index” (HSI) to indicate the stability of the alpha acids. Your homebrew store or grower’s website may provide the HSI, which can be used as an indicator that some varieties should be used more quickly than others. If not, the chart of data from S.S. Steiner — on this page — indicates the relative stability of several common hop varieties when stored at room temperature.
If the brewer stores the hops in an airtight container in a cool environment, protected from light and humidity, the rate of deterioration slows considerably. Alpha acid loss rates stated in the table are not representative of properly stored hops, which may be less than 10%. Producers may protect their hops from deterioration in sealed foil bags that have been vacuum-packed or filled with an inert gas like nitrogen. Once opened, the rate of deterioration will increase. Because pellet hops are tightly compressed, their alpha acids receive less exposure to air than whole leaf hops, and the pellets are, therefore, more stable. Storing hops in the freezer in a gas impermeable container or bag with as much air removed as possible is a good way to store hops.
While the homebrewer cannot know the degree of deterioration of alpha acids in the hops at the time they are purchased, he or she should be aware that fresh, properly stored hops more closely reflect the alpha acid rating on the package than improperly stored hops from last season. (Note: growers harvest hops in late summer and new harvests come to market around the first of the year). Be aware that the predictive ability of the preceding calculations may decline as the age of the hops increases, especially if the hops are poorly stored.
Utilization Rate
The utilization rate is the amount of isomerized alpha acids that end up in your beer divided by the amount of alpha acids added to the kettle. Depending on a number of factors, you may extract and isomerize from 0 to 40% of the alpha acids from your hops. (BYO’s standard assumption in our recipes is that pellet hops yield 25% of their alpha acids in a one-hour boil.)
The primary factor affecting the utilization rate is the length of time they are heated. The bitter alpha acids will not go into solution unless they become isomerized through heating. The longer they are heated, the greater the isomerization rate. As such, adding your hops to the boil at the correct time is important if you wish to control your beer’s bitterness. Keep in mind that changes in the way you chill your wort affect the amount of time the hops are heated. For example, if you switch from an immersion chiller to a counter flow chiller, you may alter the final IBUs of your beer. An immersion chiller cools the entire wort at (roughly) the same time. A counterflow chiller leaves an ever decreasing amount of hot wort in the kettle as chilling proceeds. Thus, your beer may taste more bitter if you switch to a counterflow chiller, especially if you use a lot of late-addition hops.
There are many other factors that affect the utilization rate. A number of different authors provide tables that suggest various utilization rates dependent on hop form (pellet or whole hop), boiling time and wort gravity. (For example, a widely used set of tables is found in Randy Mosher’s The Brewer’s Companion published by Alephenalia Publications, Seattle. Likewise, computer programs such as ProMash allow users to select utilization tables based on the work of Rager, Tinseth and others.) Homebrewers should note that these tables are educated estimates based on various factors listed above, but they have not actually been measured on their systems. To really determine your utilization rate, you would need to brew a series of test brews, have your beers analyzed for IBU content and calculate your utilization factor by rearranging the terms of the equation on page 32.
In the absence of constructing your own utilization table or equations for your system, a homebrewer should simply choose one particular utilization method and stick with it. Although the method you pick may not yield the actual utilization numbers for your system, at least it will be consistent.
Boil Intensity and Volume
The intensity of the boil is another factor affecting utilization. Applying the same amount of heat to the kettle from brew to brew will help ensure the same isomerization and evaporation rates over a specified period. Brewers should set the heat source to a consistent setting if possible.
It is good practice to boil the wort for a short time (e.g. 15–20 minutes) before adding the hops because the initial, unhopped boiling gives the coagulable proteins an opportunity to bind with harsh husk tannins instead of hop tannins. During this “pre-boiling” period, the brewer can determine the evaporation rate if the brewer has not already established a rate from prior brews. For example, if 0.25 gallons evaporate in the first 15 minutes, 1 gallon will evaporate during one hour.
A consistent boil intensity will also allow homebrewers to hit the same target gravity brew after brew, and this consistency will help them get closer to their bitterness target.
The amount of alpha acids isomerized during the boil is going to be diluted in a certain volume of wort. If you end up with less wort than you expected, your wort will be more bitter than predicted. Conversely, if you end up with more wort than expected, it will be less bitter.
To ensure that your target gravity can be hit consistently, you should calibrate both your kettle and fermenter. A “dip stick” with markings every 1/4 or half gallon is what many homebrewers use to determine how much wort is in their their kettle. Likewise, bucket fermenters or carboys can be calibrated by adding one-half to one gallon of water at a time and marking the outside of the fermenter with a permanent marker or masking tape. Don’t rely on calibrations printed on your bucket (or the fact that your carboy is “full”) to measure your volume.
Verifying Your Results
A homebrewer can perform an informal test of the accuracy of their predictions with side-by-side tasting of beers of similar known IBU and alcohol content. (Note: The perception of bitterness decreases with increasing beer gravity. Comparing bitterness is best done with beers of similar style and original gravity.)
Conclusion
Homebrewers can calculate IBU predictions for their beers, but most lack the means to actually measure their IBUs. These calculations are valuable tools, but require repeatable procedures and attention to detail to allow the brewer to actually land near the desired target. In addition, obtaining accurate values for the variables used to predict IBUs is problematic. At best, a homebrewer may be fairly consistent in producing a predictable amount of bitterness, even if his estimation of IBUs is off a bit. However, if you know what a 45 IBU beer tastes like on your system, who cares if it’s exactly 45 IBUs or not? By understanding the variables involved, you can manipulate the flavor of your homebrew, which is far more important than being numerically accurate. And remember, subtle nuances are part of what make homebrewing such an interesting adventure.
